This invention relates to the preparation of zinc alkyls by the chain growth reaction of a lower olefin, especially ethylene, with a lower molecular weight zinc alkyl and more specifically to a chain growth process on zinc, catalysed by a catalyst system comprising a group 3–10 transition metal, group 3 main group metal, lanthanide or actinide complex, and optionally a suitable activator.
The reactivity of zinc alkyls (and zinc alkenyls) with lower olefins has been reported, see Journal of Organometallic Chemistry 1973, 60, p1–10; Liebigs Ann. Chem. 1975, p1162–75; Journal of Organometallic Chemistry 1981, 221, p123–130. Di-tert-butyl zinc reacts with 2 equivalents of ethylene between −20 C and 75 C to give bis(3,3-dimethylbutyl)zinc, each zinc alkyl group effectively inserting only one ethylene molecule. Dialk-2-enylzinc compounds add to 2 equivalents of ethylene to give the dialk-4-enylzinc compounds, each zinc alkenyl group reacting with only one ethylene molecule. A second type of reaction between an alkyl zinc species and an olefin also is known. It involves an α-haloalkyl)zinc intermediate reacting with an olefin to produce a cyclopropane product and is frequently referred to as the Simmons-Smith procedure [see J. Am. Chem Soc. 81, 4256 (1959); Ibid, 86, 1337, (1964); Org React., 20, 1–131 (1973).]
There have been no reports of the preparation of zinc alkyls by reaction of a lower olefin with lower molecular weight zinc alkyl, where more than one olefin inserts into an alkylzinc bond or where the chain growth process is catalysed by a catalyst system. These types of reactions would be examples of stoichiometric chain growth, the catalysed version is known in the field as catalysed chain growth. (See eq. 1, M=Zn)
                                              ⁢                                            R              ⁢              —              ⁢              ML                        n                    +                                    pC              2                        ⁢                                          H                4                            ⁢                              ⟶                Catalyst                            ⁢                                                R                  ⁡                                      (                                                                  CH                        2                                            ⁢                                              CH                        2                                                              )                                                  p                                      ⁢                                          —ML                n                            .                                                          Eq        .                                  ⁢        1            
Catalysed chain growth of ethylene has been demonstrated for aluminium alkyls (M=Al in eq. 1), where an activated metallocene compound acts as the catalyst system. This is described in U.S. Pat. No. 5,210,338 and U.S. Pat. No. 5,276,220. According to the process described in U.S. Pat. No. 5,210,338, a catalyst system comprising metallocene halo-complexes of zirconium and hafnium and related complexes in combination with methylaluminoxane (MAO) produce aluminium alkyls, where the ethylene chain growth products are best described by the Schulz-Flory statistical distribution; polyethylene is a persistent co-product. The Schulz-Flory distribution is described by the formula χp=β/(1+β)p, where χp is the mole fraction containing p added ethylenes and β is the Schulz-Flory distribution coefficient. According to the process described in U.S. Pat. No. 5,276,220, aluminum alkyl chain growth is catalysed at mild temperatures and pressures with a catalyst system comprising an activated actinide metallocene compound. In addition, the ethylene chain growth products from the process described in U.S. Pat. No. 5,276,220 provides a Poisson-like alkyl chain length distribution and avoids formation of a polymeric co-product. A Poisson chain length statistical distribution is described by the formula χp=(xpe−x)/p!, where χp is the mole fraction with p added ethylenes and x is the Poisson distribution coefficient equal to the average number of ethylenes added per Al—C bond. As described in these patents, the catalysed ethylene chain growth processes with aluminium alkyls operate at dramatically lower pressures and temperatures than does the non-catalysed, stoichiometric ethylene chain growth on aluminium alkyls (100–200° C., 2000–4000 psi ethylene).
A number of ethylene chain growth processes on aluminium alkyls have been found to be particularly useful in the production of linear alpha-olefins and linear alcohols. Alpha-olefins can be generated from alkyl chain growth on aluminium, by displacement of the olefin product with ethylene either simultaneous with the chain growth reaction to yield a Schulz-Flory-like distribution of products or in a second, separate step. It is found that the catalysed chain growth process gives more highly linear alkyl groups than those produced under the more forcing conditions required to effect the uncatalysed chain growth reaction.
However, in certain instances, the physical and chemical characteristics of the aluminium alkyl species present in the processes described above limit the usefulness of the known catalysed chain growth processes. Aluminium alkyl compounds form compositionally complex monomeric and dimeric species that can be difficult to separate from one another or from the product olefins. They can react with product olefins to make unwanted by-products and they are highly reactive, even with relatively unreactive chemicals such as carbon dioxide, and may, over time, inactivate the metallocene chain growth catalyst systems, thereby greatly increasing their cost.
It is therefore desirable to develop catalysed chain growth processes that do not possess the limitations of the known processes using aluminium alkyls, or for which the limitations are substantially lessened.